The present invention relates to optical devices. In particular, the present invention is related to switching devices and logic implemented using photonic optical devices.
The current generation of computers utilizes a plurality of electronic transistor components. These transistors modulate the resistance to the motion of electrons (and thus current) in order to accomplish a wide variety of switching functions. Transistor electronic action controls or affects the motion of a stream of electrons through "transfer resistance" via the action of another stream of electrons.
Electronic transistors are typically fabricated using semiconductors such as Silicon (Si), and to a far less extent Gallium Arsenide (GaAs). Computing functions are performed by such electronic transistors integrated or grouped together as logic circuits on a very large scale with high device density. Due to various reasons discussed below, however, electronic transistor computing is ultimately limited to maximum data clock speeds of a few GHz in present implementations.
Semiconductor electronic switches generally are thought to have theoretical upper limits on their performance. Achievable minimum switching times are thought to be in the tens of picoseconds (10-20 ps), while minimum achievable switching power consumption and operational energy are thought to be around 1 microwatt (1 .mu.W) and tens of femto-joule (10-20 fJ) levels, respectively. Such levels imply high frequencies of operation may be possible for electronic computing.
Dense, high-frequency electronic circuit operations utilizing such electronic transistors present several persistent problems and complexities that, whether surmountable or not, are issues of concern to circuit designers. Even though electronic transistors that can operate at faster than tens of GHz do exist, the problems of electromagnetic interference, radiation, and parasitic capacitance in dense circuits limit the clock speed of electronic computers to a range of a few GHz. As the signal wavelength through the circuit becomes comparable to the circuit size, the electronic circuit will act as an efficient antenna radiating radio frequency energy. Electromagnetic signal interference or "crosstalk" is also a significant concern in these electronic circuits. High-frequency electronic circuits can suffer seriously from the problems of electromagnetic interference and radiation. Also, parasitic capacitance problems can plague the operation of a complex high-frequency electronic circuit.
It is thought that an optical circuit for which the signals are carried by light instead of electrical current may be used to eliminate the problems involving electromagnetic interference. Indeed, some present optical communication networks do utilize light transmission for portions of the network in order to increase speed and decrease interference. The problem, however, is that in order for an optical circuit to do useful computational functions, there must be a way to switch optical signals using other optical signals. There have been attempts to construct switches that partially use light beams to switch light beams in an attempt to increase speed. In such attempts, switching an optical beam with another optical beam typically involves electronics to translate an optical signal at some point to an electrical signal which is then returned back to an optical signal at a subsequent time. Such optical communications are not "all-optical communications" and typically interface with or involve electronic componentry. All-optical communications would reduce or eliminate the complexities inherent in the inclusion of electronic elements.
There have been various attempts to switch light with light without the use of electronics. A typical method of switching one light beam via another light beam utilizes a Mach-Zehnder interferometer with a nonlinear optical medium. An exemplary Mach-Zehnder Interferometer 100 is illustrated in FIG. 1. The Mach-Zehnder Interferometer 100 of FIG. 1 includes a pair of mirrors M1102, M2104 and a pair of 50 percent beam splitters BS1106, BS2108. A Signal Beam Input 110 input into the Interferometer 100 is split into a pair of beams 112, 114 via the 50 percent beam splitter BS1106. The beams 112 and 114 are recombined at the beam splitter BS2108 to form a pair of resultant beams. Signal Beam Output A 116 and Signal Beam Output B 118. If the beams 112 and 114 face equal optical path lengths as the beams 112 and 114 traverse the upper and lower arms, respectively, of the Interferometer 100, then the beams 112 and 114 will constructively interfere to become Signal Beam Output A 116 and destructively interfere to become Signal Beam Output B 118. Hence, in this event, no signal beam will be output as beam 118, while the full combined signal beam will be output as beam 116.
A Nonlinear Refractive Index Medium 120 of length Lm, known to those in the art as an optical Kerr medium, is positioned in the upper arm of the Mach-Zehnder Interferometer 100, as shown in FIG. 1. A Control Beam Input 122 with a polarization orthogonal to that of the beam 112 is introduced via a polarization beam splitter PBS1124. The Control Beam Input 122 propagates through and exits the medium 120 and is output from the Interferometer 100 via a polarization beam splitter PBS2126. The medium 120 has nonlinear optical properties, in that exposing the medium 120 to a strong light beam (in this case the Control Beam Input 122), can alter the refractive index of the medium 120. When the Control Beam Input 122 is on, the refractive index of the medium 120 will change according to the optical intensity, which is proportional to photons per unit time per unit area, of the beam 122. The refractive index of medium 120 can increase or decrease, which in turn causes the beam 112 in the upper arm of the Interferometer 100 to undergo an additional phase shift. This phase shift causes the destructive interference of the beams 112 and 114 at the beam splitter BS2108 to become constructive in forming Signal Beam Output B 118. Similarly, the phase shift causes the constructive interference of the beams 112 and 114 at BS2108 to become destructive in forming Signal Beam Output A 116. This phenomenon leads to a net switching of signal output from beam 116 (A) to beam 118 (B). When the Control Beam Input 122 is viewed as a second input signal to the Interferometer 100, this dual input, dual-output all-optical switch can be viewed as performing optical logic operation equivalent to an "AND" gate used in the electronics realm.
The Mach-Zehnder devices such as interferometer 100 can achieve all-optical switching, but due to the lack of materials with a sufficiently high nonlinear refractive index, switches of this variety typically suffer from a number of problems and drawbacks. First, the device size (indicated by Lm in FIG. 1) is large. For a medium with a reasonably high nonlinear refractive index, a device length of 1 centimeter (1 cm) or longer is needed, assuming a control power on the order of hundreds of Watts. The large size of the device clearly prohibits their use in large-scale optical logic circuit integration. Second, the switching power required is very high, in that a control power of hundreds of Watts or more is required to operate the device at high speed. Third, the speed of switching is slow if the switch is operating at close to the atomic resonant frequency of the medium. While the nonlinear effect can be substantially higher when operated at close to resonance, thereby reducing the switching power, the speed of the switching operation will be slow due to real carrier excitation in the medium limiting the switching speed to below the hundreds of megahertz for a semiconductor medium. Fourth, the Mach-Zehnder device is very sensitive to device design parameter variations and vibration because of the dependence of the device on the optical path-length balance between the two arms of the interferometer, as described above.
Other variations of all-optical switching devices exist, such as one device (not shown) that uses a cavity to enhance the intensity in a medium or to achieve optical bi-stability. This device also suffers from one or more of the problems and/or drawbacks listed above with regard to the Mach-Zehnder device. These problems make the current all-optical switching devices impractical for applications to form large-scale or dense optical logic circuits. In fact it is often quite challenging to cascade even a few of the current all-optical switching devices to work together.
In order for computers to perform faster, and to circumvent many of the complexities that accompany electronic transistor computing at increased speeds, new compact technology must be developed. It would be advantageous to provide an all-optical logic circuitry or device family capable of improved speed, of implementation at high or very high density due to smaller device sizes, of operation at lower switching energy and power consumption levels, and improved immunity to device variations.